1. Field of the Invention:
The present invention relates generally to a thermal switch suitable for use in preventing overheat or an overcurrent from occurring in either a heat generating device such as a hair dryer or a dish dryer or a small-capacity electrical motor used in, for example, an electrical washing machine or an electrically-powered shutter. More specifically, the present invention relates to a small-size thermal switch in which, after a bimetallic element deflects in a reverse direction and opens the circuit of the thermal switch, the bimetallic element is self-held in such a reversely deflected state and the self-held state of the same is maintained until a predetermined time period has passed or until an electrical power source has been switched off. Incidentally, the term "thermal switch" used herein embraces a thermostat, an overcurrent protector or the like.
2. Description of the Related Art:
Japanese Patent Laid-open Nos. SHO 54-140977 and SHO 62-222529 disclose conventional types of thermal switch having a so-called self-holding function. The self-holding function is such that, after a bimetallic element deflects in a reverse direction and opens the circuit of the thermal switch, the bimetallic element continues to be heated and self-held in its reversely deflected state, the reversely deflected state of the bimetallic element being held until an electrical power source is switched off. Such a conventional type of thermal switch has a structure in which, when the bimetallic element deflects in the reverse direction and the contact is opened, a current flows in a resistance-type heat generating element so that the heated state of the bimetallic element is held by the heat generated by the resistance-type heat generating element.
However, the related art thermal switch having the above-described structure involves the following problems. Since it is necessary that an additional resistance-type heat generating element be connected between terminals, a complex structure is needed when compared with the structure of a normal type of thermal switch. As a result, the manufacturing process becomes complicated compared with that of thermal switches of the type which does not have a self-holding function. Furthermore, the positional relationship between the resistance-type heat generating element and the bimetallic element may hinder efficient transfer of heat from the resistance-type heat generating element to the bimetallic element.
Another type of thermal switch is known in which, in a normal state, the surface of its bimetallic element which consists of a metal having the higher coefficient of expansion is maintained in contact with a movable-contact leaf spring. In a case where such a thermal switch is incorporated in an automobile or the like, a significant magnitude of vibration may be applied to the thermal switch itself, with the result that the stability of the thermal switch is lowered to a remarkable extent.